But you don't have an entitlement to speak on someone else's property.

Exactly. Social media moderation is no different than Waffle House or Dennys asking you to leave because your disruptive or antisocial behavior is freaking out the other patrons. For the less public example like blocking someone from your DMs, that's no different than telling someone they aren't welcome in your home for calling you a racial slur or whatever. For something like a subreddit where it's community self moderation rather than just the website owner admins, it's like a model railroad club or car club kicking a member out for toxic behavior. In the US the 1st ammendment also covers not just the government not being able to censor you but also the right to freedom of association.

You have a fundamental right to freedom of speech, and no one, whether they are government or someone else, can stop you speaking.

Important to note that this doesn't mean other people don't have a right to tell you to shut the fk up, shame you, or shun you for your speech. Freedom of speech goes both ways (other people have the right to speech back to you, good or bad) and that means you have to deal with the social consequences of your speech.